Method of applying a hard-facing material to a substrate

ABSTRACT

An improved method of applying a particulate material to a substrate, includes the steps of: removing impurities from a surface of the substrate; forming a coating composition having a bonding material and at least one particulate material; applying the coating composition to the substrate surface; and creating a diffusion bond between the substrate, bonding material and particulate material for generating a continuous interface between the substrate surface and particulate material such that the change in mechanical properties between the substrate and particulate material occurs in a direction normal to the plane of the substrate surface, thereby minimizing residual strain and coefficient of thermal expansion mismatch between the substrate and particulate material, the surfaces of individual particles of said particulate material being chemically wetted by the bonding material. The particulate material and the bonding material comprise a layer on the substrate surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/083,059, filed on Apr. 25, 1998.

TECHNICAL FIELD

The present invention relates generally to hard-facing coatings, and,more particularly, to an improved method for applying a coating layer toa substrate.

BACKGROUND ART

The life and reliability of hydraulic cylinders often depends on havinga hardened wear coating on the interior surface of the cylinder bore.This coating provides wear resistance and a bearing capability for thepiston head and seal. The piston rod can experience significant sideloads during use, often causing a dynamic metal-to-metal contact betweenthe bore wall and the piston head. In addition, abrasive particlesaccumulate in the piston seal. After millions of inches of pistontravel, these entrained particles can dramatically degrade the borefinish, which contributes to increased internal leakage and acceleratedwear. By providing a protective hardened coating on the bore wall, thelife of the actuator can be dramatically increased.

The common way to provide such a hardened coating is to electrolyticallydeposit chrome on the cylinder bores. Although chrome provides thedesired mechanical wear properties, there are a number of problemsattendant its use. First, the electroplating process creates hydrogenembrittlement of the substrate, which reduces the fatigue strength ofthe faced material. Second, the added wall thicknesses required toreduce stress levels to the point of affording the required life, canadd significant weight to the part. Third, chrome processing isrecognized to provide an environmental hazard. In the United States, theEnvironmental Protection Agency is tightening controls on wastestreamtreatment at plating houses. This has resulted in increased platingcosts. The government has also initiated activities aimed at developingalternative coating materials. Fourth, the use of chrome platingrepresents a significant recurring cost for the actuator manufacturer.Because the coating is not applied uniformly, a build-up of chrome canprovide an excessive thickness, particularly on edges and corners, thatmust be removed. Considerable expense is incurred in grinding down theseexcessive build-ups. Still, another factor is that ongoing efforts aredirected toward characterizing the fatigue damage due to theelectroplating process, and to determine acceptable design stresslevels.

Accordingly, it would be generally desirable to provide an improvedmethod of providing a hard-facing coating on a substrate, such as thebore wall of a cylinder, in a manner that would overcome these problems.

DISCLOSURE OF THE INVENTION

With reference to the corresponding parts, portions or surfaces of thedisclosed embodiments, merely for purposes of illustration, and not byway of limitation, the present invention provides an improved method ofapplying a particulate material to a substrate. The improved methodbroadly comprises the steps of removing impurities from a surface of thesubstrate; forming a coating composition having a bonding material andat least one particulate material; applying the coating composition tothe substrate surface; and creating a diffusion bond between thesubstrate, bonding material and particulate material for generating acontinuous interface between the substrate surface and particulatematerial such that the change in mechanical properties between thesubstrate and particulate material occurs in a direction normal to theplane of the substrate surface, thereby minimizing residual strain andcoefficient of thermal expansion mismatches between the substrate andparticulate material, the surfaces of individual particles of theparticulate material being chemically wetted by the bonding material;whereby the particulate material and the bonding material comprise alayer on the substrate surface.

The impurities may be removed from the substrate surface by thermaldecomposition, chemical decomposition, electrolytic decomposition,oblation by ions, particularly by high-energy beams, ultrasonically, byfluxing, or by some other unspecified means or method.

The coating composition may be applied to the substrate surface byspraying, dipping, painting, tape-casting, or some other unspecifiedtechnique.

The coating composition may include an organic binder, and theparticulate material may include a material selected from the groupconsisting of tungsten carbide (WC), titanium carbide (TIC), vanadiumcarbide (VC), titanium diboride (TiB₂), hafnium carbide (HfC),molybdenum carbide (Mo₂C, MoC, M₃C₂) chrome boride (CrB₂), siliconcarbide (SiC), diamond hafnium diboride (HfB₂), zirconium carbide (ZrC)and tantalum carbide (TaC).

The mechanical properties between the substrate and the particulatematerial are preferable in non-abrupt, smooth, continuous transitionalong a direction normal to the plane of the interface.

The substrate may be Inconel® (a registered trademark of Inco AlloysInternational, Inc., 3200 Riverside Rive, Huntington, W. Va. 25720,U.S.A.), 15-5 PH stainless steel, or some other material.

Accordingly, the general object of the invention is to provide animproved method of applying a particulate material to a substrate.

Another object is to provide an improved method of applying ahard-facing material to a substrate.

Another object is to provide improved coating compositions for use insuch methods.

These and other objects and advantages will become apparent from theforegoing and ongoing written specification, the drawings, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a sample arrangement during sinterbondingwith the Ni-braze forming the interlayer.

FIG. 2 is a schematic view of a sample arrangement during sinterbondingwith the Ni-braze placed at the top.

FIG. 3 is a schematic view of a sample arrangement during sinterbondingwith a WC-Ni braze composite placed on an Inconel® substrate.

FIG. 4 is a photomicrograph of the WC powder obtained from Kennametal.

FIG. 5 is a photomicrograph of the WC powder obtained from Ostram andidentified as Osram 1.

FIG. 6 is a photomicrograph of the WC powder obtained from Osram andidentified as Osram 2.

FIG. 7 is a photomicrograph of the WC powder obtained from Dow ChemicalCompany.

FIG. 8 is a photomicrograph of the Ni-167 braze alloy.

FIG. 9 is a photomicrograph of the Ni-363-2 braze alloy.

FIG. 10 is a photomicrograph of a tape-case sheet fabricated withLupasol PS.

FIG. 11 is a photomicrograph of a tape-cast sheet debound at 650° C. forone hour in a nitrogen atmosphere.

FIG. 12 is a schematic of tape-cast fabrication.

FIG. 13 is a photomicrograph showing the cross-section of a WC-nickelbraze composite bonded to an Inconel® substrate.

FIG. 14 is a photomicrograph showing interface cracking due torestrained shrinkage.

FIG. 15 is a photomicrograph showing a sample in cross-section, anddepicting the effect of reducing the hold time.

FIG. 16 is a photomicrograph showing a sample in cross-section, anddepicting the effect of reducing the cooling rate and the hold time.

FIG. 17 is a photomicrograph showing a sample in cross-section, anddepicting a defect-free interface.

FIG. 18 is a photomicrograph of the WC-nickel braze composite coating onan Inconel® substrate.

FIG. 19 is a flow chart showing the schematic steps involved inproducing WC tape-cast material.

FIG. 20 is a flow chart showing the schematic steps involved inproducing predensified nickel braze.

FIG. 21 is a flow chart showing the schematic steps involved in cleaningthe substrate prior to application of the coating composition.

FIG. 22 is a flow chart showing the schematic steps involved inproducing WC tape-cast material on an Inconel® 600 substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., cross-hatching, arrangement of parts, proportion,degree, etc.) together with the specification, and are to be considereda portion of the entire written description of the invention. As used inthe following description, the terms “horizontal”, “vertical”, “left”,“right”, “up” and “down”, as well as adjectival and adverbialderivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”,etc.) simply refer to the orientation of the illustrated structure asthe particular drawing figure faces the reader. Similarly, the terms“inwardly” and “outwardly” generally refer to the orientation of asurface relative to its axis of elongation, or axis of rotation, asappropriate. Also, equivalent symbols have been used as shorthandexpressions of full chemical names.

Powder injection molding (“PIM”) is an established manufacturing methodfor the production of high volume components having complex geometricalforms. The material flexibility, superior mechanical properties, andprecision surface finish of PIM make it well suited for manyapplications. When the complexity of the manufacturing operation doesnot require this advanced technologies, polymer/powder mixtures can beused in other forming methods.

Binder-assisted forming processes, such as powder injection molding,have focused on operations requiring pressure to fill the die cavity.Other processes, however, have been developed that do not afford theprecision provided by injection molding, yet they borrow the concept ofbinder-assisted forming. Such processes include tape-casting forhard-facing applications, and centrifugal molding for the formation ofsimple geometric shapes.

Tape-casting was developed to deposit a single, thick layer of tungstencarbide on a superalloy substrate, where the coverage area was in excess1550 cm², with a thickness of 2.5 mm. Centrifugal forming was developedto augment a die compaction protocol used to fon-n braze preforms foraircraft engine repair. The die compaction press that was used for theapplication had a 500,000 kg [550 ton] maximum capacity. Parts beingdeveloped by aircraft engine designers would exceed this capacity.Therefore, alternate forming methods were sought. Because this processmust be compatible with a currently existing process, the alloychemistry was fixed. The final goal was to produce a compatible greenpart having a similar density. The powder preform is composed of ahigh-melting nickel-based powder and a low-melting nickel-basedmaterial.

One objective was to develop a tungsten-carbide-nickel braze compositecoating for an Inconel® 600 substrate. The entire project was dividedinto five stages of research: (a) literature survey, (b) experimentaldesign, (c) tape-cast sheet production, (d) optimization of the bondingprocess, and (e) evaluation of corrosion and wear properties of thehard-faced material.

BACKGROUND

Tungsten carbide has been extensively exploited in tool industriesbecause of its high hardness and abrasion resistance [Ref. No. 1].However, processing of tungsten carbide is difficult as it is arefractory material and cannot be densified by solid state sintering athigh temperatures. Hence, a binder phase is incorporated in the systemto facilitate densification of the material at lower temperatures. Theconsolidation of the composite is achieved by heating the powders abovethe melting point of the binder phase. Pressure-assisted sintering isused to obtain fully-densified cemented carbides with high volumefractions of tungsten carbide. The liquid formation temperature shouldbe low enough to avoid the dissolution of tungsten carbide.

The use of the binder phase is not limited to the densification process.The properties of the carbide can be tailored by on the nature andcomposition of the binder phase [Refs. No. 1,2]. The binder phaseaffects the toughness, ductility, hardness and abrasion resistance ofthe composite [Ref. No. 1-3]. The suitability of the binder phase isdictated by the mechanical properties desired for the final cementedcarbide.

Conventional cemented carbides often contain cobalt as the binder phasebecause it satisfies the desired property requirements. The binderimparts the necessary toughness and ductility, and facilitates theproduction of the cemented carbides at temperatures much lower than themelting point of the carbides [Ref. No. 3]. However, cobalt is strategicmaterial, and is therefore expensive and comparatively rare. Despiteearly attempts [Ref. No. 4] to find a substitute for cobalt as thebinder phase, production has been greatly limited to modeling inlaboratory scale.

The major restriction for the use of substitute materials arises frominadequate mechanical property generation. Iron and nickel, incombination with low-melting constituents like boron and silicon, arethe major candidate materials for the binder phase. Comparablemechanical properties with the cobalt-cemented carbide were achieved forthe Fe/Ni alloy and the alloy in combination with cobalt [Refs. No. 3,4]. The additional advantage realized was that the binder phase was heattreatable. However, addition of low-melting constituents (e.g., boron)as deemed necessary to facilitate liquid phase sintering at lowtemperatures.

Aronsson et al. [Ref. No. 1] reported poor distribution of Ni-basedbinders between the tungsten carbide particles during milling, resultingin poor densification and high porosity levels. Even distribution of thebinder is desired to facilitate liquid film formation on binder meltingbetween individual tungsten carbide grains, which enhances densification[Ref. No. 6]. This problem will be more pronounced in the currentproject at low weight fractions of the binder phase. The presence ofagglomerates of binder phase in the initial blend is harmful and hassimilar effects as poor distribution [Ref. No. 7]. The availability ofrelatively coarse binder powders will negate this issue. However, thesize has to be adjusted to combine with the carbide powder to providegood initial packing densities.

The above section summarized the binder phases which are potentialsubstitutes for cobalt. An advantage of these systems is that they areinexpensive and readily available. The microstructures obtained for thevarious systems showed excellent bonding between the binder and thecarbide phase [Refs. No. 1-5, 10].

Ni-Cr-Mo was investigated by Roebuck et al. [Ref. No. 14] as areplacement for cobalt in tungsten carbide cemented carbides to improvethe mechanical properties and oxidation resistance. The presence of Moconferred improved corrosion resistance, in both acid and alkalinemedia, together with increased toughness, for a given hardness, incomparison with equivalent tungsten carbide/cobalt cemented carbides. Itwas observed that Mo partitions to both the tungsten carbide and thebinder phase (nickel phase). In an earlier paper [Ref. No. 15], the sameauthors discussed the use of an infiltration technique to producetungsten carbide hardmetals with Co and Ni alloy binders. The methodinvolved incorporation of alloy elements into an already-dense cementedcarbide by placing the alloy addition on its surface and infiltrating at1500° C. for 0.5 hr. This way, infiltrated carbides with variouscompositions for the binder could be generated. The properties requiredfor the binder are similar to those required in liquid phase sintering,i.e., the binder should be liquid and it should wet the carbide at theinfiltration temperature. This method was successful in obtainingreasonably homogeneous microstructures of tungsten carbide with variousCo and Ni alloys as the binder phase. The binder phases studied includedpure Ni, Ni-W-C alloys. Ni₃Al, Al₃Ni, and alloys with Ni, Co, Cr, Mo, W,C and Al. Table 1 lists the binder composition, the binder content, andthe hardness of three such Ni-alloy infiltrated hardmetals.

TABLE 1 Cemented tungsten carbide with nickel-based binder phase.Binder-phase composition (wt. %) Binder Content Hardness, Ni Co Cr Mo WC Al Ti wt. % HV 30 70 15  7 1 2 1 4 — 10 1390  65 10 15 — 2 1 3 4 22883 60 20 10 2 2 1 5 — 22 869

Information on some standard compositions is given in Table 2 forcomparison, and also the effect of decreased grain size of tungstencarbide and decreased binder content in increasing hardness.

TABLE 2 Standard cemented tungsten carbide. Binder Content, TungstenCarbide Hardness, Binder Type (wt. %) Grain Size (μm) HV 30 Cobalt 60.96 1540 Cobalt 6 3.2 1230 Nickel 6 1.2 1495 Nickel 10 1.2 1190

Roebuck et al. [Ref. No. 15] clearly showed the possibility of producinghardmetals with complex nickel-base alloy binder phases. They detectedthe presence of gamma prime precipitates (mainly, Ni₃Al) in binderphases containing Ni, Cr, and Al. These precipitates are advantageousfor mechanical properties. Further, in spite of the high infiltrationtemperatures, no excessive grain growth of tungsten carbide was seen insamples with highly alloyed binder phases. This signifies that thealloying additions inhibit grain growth, perhaps due to limitedsolubility of the WC ion in the binder. Aronsson and Pastor [Ref. No. 1]reported that Cr-Ni rich binder phases are often used for good corrosionproperties. Further, they reported that compositions similar tonickel-based superalloys have also been tested with satisfactoryresults.

Nickel-based alloys are used in powder form to enhance wear andcorrosion resistance and are applied by various hard-facing techniques[Ref. No. 16]. These alloys typically contain Ni, Cr, B, Si, and oftensmall amounts of C and Fe. Both B and Si form low melting eutectics withnickel or nickel-solid solution. The lower melting point of these alloysover conventional superalloys is an advantage as it eases theprocessing. Boron and silicon act as deoxidizers and improve theproperties of the coating material and in bonding to the substrate, aswell as melting temperature depressants for Ni and Cr via the formationof eutectic liquids. Both are expected to diffuse into the substrateduring the coating procedures.

Mixtures of the nickel-based alloys (their compositions and meltingranges are presented in Table 3) with tungsten carbide, TiC, and (W,Ti)Cwere deposited upon steel samples and processed to a coating in a vacuumfurnace. They used a relatively high binder content (50-70 wt. %) as theabrasion resistance of the binder in itself was expected to be quitegood.

TABLE 3 Nickel-based alloys used for hard-facing carbides on steel. ° C.Alloy Cr S B Fe C Ni T_(s) ^(a) T_(L) ^(a) HFA1 10.0 3.0 2.25 4.25 0.45bal. 965 1180 HFA2 13.5 4.25 3.0 4.75 0.75 bal. 965 1035 ^(a)T_(s) andT_(L), are the solidus and liquidus temperatures, respectively.

Knotek et al. [Ref. No. 16] attained the best hardness and abrasionresistance at high carbide contents, low processing temperatures, andshort hold times at the processing temperatures. With increasingtemperatures and the holding times, narrow layers of eta carbides(mainly Ni₂, W₄C and traces of Fe₃W₃C) formed on the surface of thecarbides. Further, Fe from the substrate also diffused into the matrixphase. The formation of the eta carbide layers around the carbide grainscaused brittle fracture between the binder and the carbide.

The alloy HFA 2 has a higher B and Si content than HFA 1. This alloycaused more dissolution of carbides and the formation of Ni₂W₄C incomparison to alloy HFA 1. Further, in general for both alloys, theformation of the carbides was accelerated by an increase in processingtemperature and time. When mixtures with both the alloys were given thesame thermal treatment, large amounts of tungsten carbide remained, andlittle eta carbide formed in HFA 1. On the other hand, tungsten carbidedissolved almost completely in HFA 2 to form Ni₂W₄C.

The wetting between Ni (and nickel-based alloys) and tungsten carbide isextremely good. The contact angle of 0° between the two was attributedto the alloying between tungsten carbide and Ni by Knotek et al. [Ref.No. 16]. Further, they also showed the use of using (W.Ti)C compositesin avoiding segregation. Pure tungsten carbide tends to settledownwardly, while pure TiC tends to float.

The presence of carbon is of prime importance in the development ofcemented carbides. Insufficient carbon levels lead to the formation ofbrittle eta phase. The eta phase is a double carbide and its compositiondepends on the binder phase used (e.g., Co₃W₃, C for Co binders, andFe₃W₃C for iron-based binders). Its presence adversely affects themechanical properties, and is attributed to its brittle nature. Etaphase formation occurs due to instability of tungsten carbide inpresence of low carbon austenite. The tungsten carbide breaks down andreacts with the austenite to form the eta phase. The hardness is maximumfor a C content which results only in the formation of tungsten carbide.Excess carbon results in presence of free graphite, which degrades thestrength and hardness of the carbide. Moskowitz [Ref. No. 4] found theoptimum carbon content to be 4.9% for his Fe/Ni binder system. Thedesired carbon level can be ascertained by calculating the total carbon,tungsten and iron content from chemical analyses. The presence of Nireduces the carbon requirement needed to prevent eta formation. Theauthors associated no reason to this effect but it can be hypothesizedthat Ni prevents the reaction of the matrix with the carbide. Theaddition of TaC and VC increases the hardness and the abrasionresistance of the material. Their effects are more pronounced for lowbinder contents [Ref. Np. 5].

Brazing of a ceramic or a carbide to a substrate using a filler metal isoften used for different applications. Information on pertinent brazingoperations is included to provide a complete sense of the availabletechniques and suitable filler materials. In the brazing processes, thedifferential expansion and contraction between the ceramic and themetal, and the resulting effect on the joint properties, becomes a majorconcern.

Roberts [Ref. No. 17] presented the criteria necessary to producesatisfactory brazed joints between tungsten carbide (Co-cemented) andsteel for application in drilling tools. Brazing was reported to be thepreferred method of joining the carbide and its supporting mass ofsteel. Further, silver-based alloys were reported to be most popular dueto their low melting temperatures, excellent wetting characteristics,and good mechanical properties. These alloys generally contain Cu, Zn,Cd, Ni and Mn, and operate at temperatures in the range 640-840° C.Copper-based brazing alloys are also used, considering the high price ofsilver. These alloys typically contain Cu, Ni, Mn, Si and Zn, andoperate at temperatures between 870-1100° C. One of the requirements ofthe braze filler is that it is of sufficient thickness so that it canabsorb the stresses introduced due to the difference in the expansioncoefficient of steel and tungsten carbide. A silver-base alloy,Ag71Cu27Ti2, was also used as the filler metal by Chen et al. [Ref. No.18] to braze Si₃N₄ with Inconel® 600. Their work concentrated on theinterface between Inconel® and filler metal. The bonding mechanismbetween the two was attributed to the diffusion of Ag and Cu alloy intothe grain boundaries of Inconel® 600 resulting in mechanical anchoring.In another study, Kang et al. [Ref. No. 19] used Au-18% Ni and Ag-28 wt.% Cu as braze alloys, because of their ductility and oxidationresistance, to bond a Si₃N₄-based ceramic and a Fe-Ni-Co alloy. Themechanical properties of the brazed joint were reported to bereasonable.

McDermid et al. [Ref. No. 20] performed research enjoining ofreaction-bonded SiC to Inconel® 600 for application in advanced heatengines using a nickel-based brazing alloy BNi-5 (Ni-19Cr-10Si). Due totheir low melting points, silver- and copper-based braze alloys areunsuitable for such applications considering the high temperaturesinvolved. The authors used both direct brazing and composite interlayerjoining. The composite interlayer, consisting of powder mixtures ofalpha-SiC and BNi-5, was used to reduce the coefficient of thermalexpansion (“CTE”) mismatch stresses between the ceramic and the metal,which are generated during cooling. In both methods, the liquid fillermetal reacted with the free Si of the reaction-bonded SiC to form aNi-Si liquid. This liquid, in turn, resulted in the degradation of boththe reaction-boned SiC and Inconel® 600. Further, when the compositeinterlayer was used, the molten filler metal decomposed the alpha-SiCpowders to form a Ni-Si liquid which reacted with the reaction bondedSiC and Inconel® 600 layers. Thus, it was concluded that the reaction ofthe nickel-based braze alloy with the ceramic in this case prohibitedits use in joining reaction-bonded SiC and Inconel® 600.

Table 4 summarizes the properties of the various binder phases discussedin the above section.

TABLE 4 Abrasion Final Transverse Resistance Density, Rupture FactorFracture Binder Binder (% Strength (cc/rev^(a) toughness Hardness Phase(wt %) theoretical) (GPa) 10⁻³) (MPa/m^(2/3)) (kg/mm²) Fe- 15  — — 8 —91 HRA 10%Ni 316SS 6 96.3% 1.22 — 10.6 1458 316SS 9 96.6% 1.33 — 11.41398 60Fe- 6 96.4% 1.44 — 10.9 1439 10Co- 30NiMoB 60Fe- 9 97.3% 1.35 — 1.5 12.1 10Co- 30NiMoB

The sintered properties are affected by the composition of the binderphase which in turn dictates the final microstructures and porositylevels. Iron-nickel alloys serve to reduce the processing temperatureand does not react with tungsten carbide. Nickel is essential tostabilize the martensitic phase and to improve the corrosion resistanceof the developed carbide [Ref. No. 5]. In addition, the presence of ironmakes the binder system heat treatable. Fe-Ni binder inhibitscarbide-grain coarsening, resulting in fine-sized grains and producingsuperior mechanical properties as compared to tungsten carbide-cobalt[Ref. No. 3]. Stainless steel cemented carbides exhibits a finer grainsize than Fe-C-NiMoB-based systems. Hence, a higher density is achievedin the stainless steel cemented carbide [Ref. No. 10]. Farooq and Davies[Refs. No. 11, 12] further found that a narrow particle size range ofthe binder phase improved the final sintered density. The use ofstainless steel served to improve the sintered density, mechanicalproperties and the corrosion resistance of the material. The onlydrawback was associated with the high sintering temperature requirements[Refs. No. 10-12].

In general, increasing binder content improves the toughness and thetransverse rupture strength of the material while decreasing thehardness and the abrasion resistance.

Table 5 lists the properties associated with the modified binder withvarious alloy additions.

TABLE 5 Transverse Abrasion Rupture Resistance Fracture Binder BinderFinal Strength Factor Toughness Hardness Phase (wt. %) Density (GPA)(cc/rev⁻³) (MPa/m^(2/3)) (HRA) Fe-10%Ni + 25 — 4.2 36 — 4.9%C Fe-10%Ni + 5 96.3% 0.52 1% VC Fe-10%Ni + 15 — — 14.3 89 1%TaC

Vishwandham et al. [Ref. No. 13] made Al additions to Ni-based bindersystems. This resulted in the generation of superior properties due tothe precipitation of the gamma phase, which provided dispersionhardening to the matrix. Hence, an increase in the hardness and wearresistance was obtained. Prakash et al. [Ref. No. 3] found that the sameeffect was observed for additions of Mo and Ti to the system. Additionsof cobalt to Mo-Fe-Ni systems improved the mechanical properties furtherby lowering the solubility of Mo in Fe-Ni, thus increasing the amount ofdispersed phases. Mo₂C and Cr₃C₂ increased the hardness of the cementedcarbide by reducing the grain size of the material. A similar effect wasobserved for VC additions to the binder system [Ref. No. 2].

Design of Experiment

Good interfacial bonding is desired between tungsten carbide and thenickel braze to avoid tungsten carbide pull-outs during actual serviceof the material. As mentioned earlier, good wettability is the key tothe problem. Wettability can be tested by sintering a tungstencarbide-nickel braze mixture compact above the liquids temperature ofthe braze alloy (where the nickel braze alloy is present as a liquidphase only). Poor wettability results in swelling of the compact, andliquid exuding from the compact. The exuded liquid appears as smallspherical balls on the surface of the compact. This processing stephelps in isolating the suitable braze alloys for further processing.

A tungsten carbide-nickel braze composite can be developed in twodifferent ways. The first method involves sintering a mixture oftungsten carbide-nickel base mixture above the liquids temperature ofthe nickel braze to allow for densification of the material in presenceof a liquid phase. The limitation with this process is that the effectof relative size and density of the nickel braze and the tungstencarbide becomes prominent during mixing and infiltration. The nickelbraze powders are generally available in the coarser size range, asopposed to tungsten carbide which is preferred in finer sizes to enhancethe mechanical properties. This can lead to segregation during mixing.More critical are the sintering problems associated with the powdersize. On melting, nickel braze may leave behind a large pore (site ofits original occupation), and fill up smaller ones. This would lead topoor densification at the completion of the sintering process.

The other method involves fabrication of tungsten carbide-nickel brazecomposite by selective infiltration of tungsten carbide porous preformsby nickel braze liquid. Nickel braze and tungsten carbide, in the formof tape-cast sheets, are stacked one over the other and sintered abovethe liquids temperature of the nickel braze. Densification is achievedthrough capillary, and gravity-induced infiltration of nickel brazeliquid into the tungsten carbide sheet. This method is not limited bynickel braze particle size, as it is liquid prior to infiltration.Another advantage with this design is that homogenous composite can bedeveloped by controlling the pore size of the tungsten carbide tape-castsheet.

The bonding process requires the nickel braze alloy as a cementing agentbetween the Inconel® substrate and the tungsten carbide. The liquidshould be minimized at the interfacial joint between the braze alloy andthe substrate to reduce the occurrence of weak regions which can providefor interfacial cracking. Three schematic designs are depicted in FIGS.1-3. In FIG. 1, a predeveloped tungsten-carbide-nickel braze compositeis laid over the Inconel® substrate, and is heated above the nickelbraze liquidus temperature. Cementing is achieved through reactionbetween the nickel braze in the composite and the Inconel®. FIG. 2depicts the nickel braze as the interlayer, with tungsten carbide at thetop and the Inconel® at the bottom. On heating above the liquidstemperature, the nickel braze melts, wets the Inconel® and infiltratesthe tungsten carbide layer. The limitation with such an arrangement isthat the amount of liquid at the interface cannot be controlled. FIG. 3is an alternate arrangement to the earlier design where the tungstencarbide forms the interlayer with the nickel braze at the top and theInconel® at the bottom. In this lay-up procedure, gravity aids in theinfiltration process and the excess liquid remains at the top. Both theabove processes yield the composite and the hard-faced Inconel® in asingle operation. The following points need to be noted: (1) the processinvolves melting of the nickel braze alloy, (2) bonding is achievedthrough the formation of eutectic liquids between the braze alloy andthe Inconel®, and (3) infiltration of nickel braze liquid into tungstencarbide by capillary action and/or gravity.

The development stage involved a sequential analysis of the problem athand, starting from powder selection to optimizing thermal cycle for thehard-facing process.

The criterion for powder selection includes evaluation ofcharacteristics such as shape, size, density, flowability,compactibility, etc., which characteristics are responsive to theoperations carried out on them. Powders of different size range whereevaluated for their ease in processing (e.g., production of tape-castsheet, packing, sintering, etc.). The selection of the Ni-braze powderswere based on the melting temperature range and their wettability withrespect to WC-powder.

Sample tungsten carbide powders were obtained from Kennametal, OsramSylvania, and Dow, while the nickel braze alloys were acquired fromPraxair and Amdry. The powders were characterized for their size, shape,and densities using standard available equipment. A brief description isgiven below on each of these processes.

The particle size of the powders was measured using the aerosizer whichemploys the time of flight of particles to give an equivalent sphericaldiameter. Table 6 shows the compositions of the two nickel braze alloys.

TABLE 6 Braze Alloy Composition Ni-167 Ni-0.8C-4.5Si-14.5Cr-3.3B-4.5FeNi-363-2 Ni-4.2Si-7Cr-3B-3Fe

Table 7 reports the particle size of the powders. The Osram and Dowcontained agglomerates. Data available from the companies indicate thepowders to be submicron in size.

TABLE 7 Particle Size Particle Size (μm) Reported (μm) Powders d₁₀ d₅₀d₉₀ d₅₀ Remarks tungsten carbide Kennametal 7.4 12.5 17.6 10 dispersedOsram-1 4.1 7.4 9.4 agglomerated Osram-2 1.1 1.6 2.2 agglomerated Dow-16.6 12.9 19.0 0.8 agglomerated nickel braze alloy Ni-167 48.9 77.9 117.3— coarse Ni-363-2 45.2 71.2 108.5 — coarse

The particle shape was observed under a scanning electron microscope.Photomicrographs of the powders are shown in FIGS. 4-9. The nickel brazepowders are spherical in shape as they were gas atomized. The Kennametaltungsten carbide powder has an angular irregular shape. The othertungsten carbide powders were agglomerated due to their fine size. Thesepowders had thermally bonding due to their fabrication technique. Theagglomerates can be broken by rod milling or ball milling.

Powder density was measured using a helium pycnometer. This instrumentmeasures the true volume of a powder. By dividing the mass by themeasured volume, a density can be calculated. This calculated valueshould be close to the theoretical density of the powder. Any open porespresent are not measured by the system. Pressurized helium is used tomeasure the volume of pores in a powder of known mass, in a samplecontainer of known volume V_(s) but unknown powder volume V_(F). Thecontainer is pressurized to a pressure P and then evacuated by allowingthe gas to expand in a cell of known volume V_(s). The drop in pressureis recorded and the ideal gas law is employed to acquire the powdervolume, and, hence, the density of the powder. Table 8 reports thepycnometer density of the powders.

TABLE 8 Powders Pycnometer Density (g/cm³) tungsten carbide Kennametal15.54 Osram-1 15.58 Osram-2 15.3 Dow-1 15.73 nickel braze alloy Ni-1677.7 Ni-363-2 7.95

The composite fabrication proceeded through the two methods discussedearlier. The tungsten carbide and the nickel braze powders were mixedtogether in the first set of experiment. However, though goodwettability was observed between the nickel braze and the tungstencarbide, the final density obtained was poor because of poor mixingbetween the nickel braze and the tungsten carbide and segregation effectdue to particle size difference (nickel braze-78 μm, and tungstencarbide-17 μm). To enhance good intimate contact between the tungstencarbide and the nickel braze, the powders were rod-milled for a longtime (4 hrs) and sintered. Similar density as in case of mixing wasobtained. This explained the prominent effect of particle sizedifference and this method was discontinued for future processing.

The second set of experiments using the simple tape-cast lay-up schemeproved effective and was employed for future experiments. In thefollowing sections, the processing schedules in this part of experimentshave been outlayed with emphasis on generation of tape-cast sheet andthe important parameters associated with them.

Tape-cast fabrication proceeded through three main stages. The firststage involved the selection of a suitable binder system that wascompatible with the powders and yields a flexible homogenous sheet. Inthe second stage, the composition of the sheet and fabrication routeswere considered to allow for maximum solids loading (i.e., volumefraction of tungsten carbide) without sacrificing the flexibility andthe uniformity of the sheets. In the final stage, the various mixingroutes were investigated in view of the homogeneity of the tape-castsheet. The following sections briefly discuss each stage individually.

For generation of a tape-cast sheet, the additive system primarilycomprises binder, plasticizer, solvent, and a surfactant (if required).The binder provides the necessary bonding between the powder particleswhich facilitates room temperature handling. The plasticizer is used toinduce flexibility in the sheet which allows it to conform to substrateswith a slight curvature. Surfactant is added to disperse agglomeratesadherent due to weak van der Waal forces or presence of moisture. Oneimportant aspect of the additive system is that its residual ash contentneeds to be low. This becomes more critical for systems, like tungstencarbide, which require close carbon control. An additional criterionspecific to this system is the selection of a water-based binder system.

The binder system employed needs to blend in with the tungsten carbidepowder to provide for homogeneous tape-cast sheets. Based on therequirements, three binders were finally selected after preliminarytesting, which involved hand mixing of the binder system with the powderand checking for flowability of the suspension. In addition, the binderswere burned under argon in a thermogravimetric analyzer to record theresidual ash content. The binders which were selected after preliminarytest were Duramax B-1007, acquired from Rohm and Haas, and Lupasol PSand Lupasol SKA, both obtained from BASF Corporation. The results fromthe initial evaluation of binders are reported in Table 9.

TABLE 9 Solids Molecular Viscosity Mixing Binder Vendor Loading Weight(cps) Suitability % Residue Burnout Duramax Rohm 33% — low poor 1 cleanand Haas Lupasol BASF 20% 2,000,000 500-1000 good 10 dirty SKA LupasolPS BASF 33% 750,000 700-2100 good 1 clean- moderate Lupasol BASF 99%25,000 100,000- good 1 clean- water-free 250,000 moderate

As demonstrated in the above table, Duramax and Lupasol PS demonstratedthe best properties for use in the cast sheet fabrication. They have alow residue ash content, indicating a clear burn-out. They alsogenerated uniform tape-cast sheets. FIGS. 10 and 11 show a scanningelectron microscope of powder-binder distribution in the tape-castsheet. The use of Duramax was later discontinued.

Preliminary preparation of tape-cast sheets was carried out using handmixing. This was done to test for suitability of the binder phase todisperse with the metallic powder. The time required for such operationwas essentially long and resulted in non-homogeneous tape-cast sheets.One major problem encountered was formation of air bubble during mixing,which resulted in regions of large pores. This affected the finalcomposite, with the development of isolated liquid pools at these sites.

An alternate and efficient way of mixing is vacuum mixing forpreparation of the slurry for tape-casing. This kind of mixing can becarried out in a Whip Mixer™ having a plastic bowl (300 ml capacity)with a paddle mounted on a rotary shaft. The bowl holds thebinder-powder system and can be connected to vacuum. The whole mixingprocess requires about 30-60 seconds and it relieves the mix of airbubbles and other gaseous contaminants. The advantage of such a mixtureis two-fold: first, it removes formation of air bubbles during mixing byemploying vacuum, and second, the mixing time is greatly reduced to oneor two minutes depending on the amount of slurry. The critical issuesduring mixing are the density difference between the binder and powder,and the viscosity of the binder. Mixing under vacuum results in densityseparation of the powder and the binder This adversely affects themixing process, and, hence, formation of a suitable slurry. This problembecomes most prominent for low viscosity binders which do noteffectively disperse with the powder during vacuum. Having a lowviscosity, Duramax has a tendency to separate under vacuum-inducedmixing. This problem was not obvious during hand mixing, and, hence, useof Duramax was subsequently discontinued.

The binder composition used in the creation of tape-cast sheets islisted in Table 10.

TABLE 10 Density Additive System Material (g/cm³) Remarks binder LupasolPS 1.03 clean burn out solvent water 1.00 — plasticizer dibutylphthalate 1.04 (small additions) (DBP) surfactant duracan (ammonia —(small additions) dispersing agent)

The composition of the powder-binder mixture in the slurry is shown inTable 11.

TABLE 11 Material Solids Loading Binder Water Plasticizer tungstencarbide 48% vol.   40% vol.     5% vol. 7% vol.   Ni braze 60.7% vol.24.9% vol. 10% vol. 4.4% vol.

The casting substrate is of crucial importance as it defines the surfacefinish and the thickness of the tape-cast sheet. This essentiallydictates the surface smoothness of the final composite. The castingsurface should have a glassy finish, and should demonstrate poorwettability to the slurry so as to ensure easy release of the sheet ondrying. The use of porous substrates aid in drying and also contributesto the increase in solids loading through rearrangement on drying. Thisgreatly reduces the drying time and higher volume fractions of thetungsten carbide can be achieved.

Preliminary studies were carried out using glass substrates, withmounted slides which controlled the tape-cast sheet thickness. Thetape-cast sheet had long drying times, and the problem of mold releasewas evident. Polymer sheets were subsequently used as substrate whichsolved the mold release problem but the drying time was still extensive.Many porous sheets were investigated to determine their suitability ascasting substrates.

Ultracel™ is a concrete material that is reinforced by fiberglass. Thesubstrate was cast at Penn State University using a powder-water mix.The cast material has a smooth surface finish. However, as it isdifficult to control, the pore size is non-homogenous. Sheets that weretape-cast on this substrate had reduced drying time, increased solidsloading, and a smooth surface finish. The limitation with this substrateis that it demonstrates poor mold release. The reason for this isassociated with the large pores present in the ultracel substrate, whichsucks in powder along with the water during drying (pore size is greaterthan powder size). Ultracel also had a tendency to leave a trace on thecast sheet. External impurities cannot be tolerated, as the surface ofthe sheet needs to be clean for effective bonding. Hence, the use ofthis substrate had to be discontinued. The next alternative which wasinvestigated for casting was porous alumina substrates. These aluminasheets are commercially available, and are used for slip casting, avariant to the tape-casting process. The alumina sheets are castsimilarly to the ultracel, and then presintered to provide sufficientstrength to the sheet. This also does not permit any trace marks to beleft on the tape-cast sheet on removal. No mold release problem wasencountered with the use of this substrate. However, the pore sizecontrol is very poor in these sheets, and the presence of large poreswas attributed to air bubble formation during its casting. Good poresize control for casting of tungsten carbide tape-cast sheets limits ituse.

Hydrophilic polyethylene sheets are commercially available and arepolymer sheets with fine-sized pores. The pore size ranges from 15-45μm. They have excellent surface finish and close pore size control. Asthe name itself suggests, the polymer sheets are hydrophilic (i.e.,water loving). The polyethylene in itself is hydrophobic (i.e., waterhating), and are treated with surfactants to induce hydrophiliccharacteristics. Being a polymer, the sheet demonstrated good moldrelease and leaves no trace on the material being cast. Water absorptionis practically instantaneous, and, hence, the slurry dries once pouredeven before it could be cast. This sheet is costly, as compared to thealumina sheet, but provides much better surface finish, porehomogeneity, and good mold release. This substrate was finally selectedfor casting the tungsten carbide tape-cast sheet.

Table 12 lists summarizes the characteristics of the various substratesused.

TABLE 12 Pore Size Substrate Pore Size Distribution Nature Mold Releaseglass dense — none poor ultracal coarse wide hydrophilic poor polymerdense — none good sheet polyethylene fine narrow hydrophilic good (15-45μm) alumina fine-coarse wide hydrophilic good

The tape-cast sheet was fabricated on the hydrophilic polyethylenesheet. The thickness of the sheet was controlled by two plexiglassslides which were mounted on the two ends along the width of thesubstrate and extended parallel along the length direction. This ineffect formed a cavity which held the slurry. A doctor blade was runover the slurry while resting on the glass slides mounted on thesubstrates. The thickness of the sheet was controlled by the thicknessof the glass slides, as schematically shown in FIG. 12. To makeprocessing simpler, the hydrophilic sheet is pretreated with water inorder to introduce a back pressure while casting the tape-cast sheet.This allows sufficient time for the slurry to be cast without drying.The pores present in the substrate removes water by capillary action andconsequently increases the solids loading through particlerearrangement.

The bonding study was the most extensive as the cleaning procedure,thermal cycle, sample layout, and annealing treatment had to beoptimized to give a fully-dense tungsten carbide composite and adefect-free interface. This particular section account for the attemptsmade in this phase of study.

It is critical for the surface of the Inconel® to be free of any oxidesor contaminants to ensure poor bonding between the tungstencarbide-nickel braze composite and the Inconel®. The Inconel® wassubjected to thermal treatment to burn of any volatile constituents andreduce the oxides. Inconel® was heated at a heating rate of 10° C./minto 1000° C. in hydrogen with a dwell of 1 hour. The temperature wassufficient to reduce any oxides present in the material. After thethermal treatment, the Inconel® was cleaned with acetone prior to thebonding process.

The nickel braze was pre-densified prior to the bonding process. Thereason attributed to this is that the nickel braze alloy is verysensitive to impurities, which tend to offset their liquids to hightemperatures. The binder present in the tungsten carbide tape-cast sheettends to penetrate the pores in the nickel braze tape-cast sheets, andoffset its melting to higher temperatures. Hence, predensification isdesired for proper melting and composite development.

The thermal cycle was optimized in terms of the bonding temperature andtime of hold. The bonding temperature was fixed based on the liquidstemperature of the nickel braze alloy. The nickel braze alloydemonstrated a melting temperature range of 930-970° C. Superheat wasprovided to lower the viscosity of the nickel braze, which decreasesexponentially with temperature. Based on these requirements the bondingtemperature was fixed at 1100° C. The hold time at the bondingtemperature is also critical as an optimum limit is required. The timeprovided should be enough for complete infiltration of the tungstencarbide sheet by the nickel braze liquid. However, the reaction betweenthe Inconel® and the nickel braze liquid should be controlled at theinterface to avoid excess liquid formation. The liquid formation due toa eutectic formation between the Inconel® and the nickel braze isdiffusion controlled, and the thickness of interface varies as squareroot of time. Hence, the hold time was optimized.

The sample layout schemes are illustrated in FIGS. 1 and 2. As discussedearlier, by keeping the nickel braze liquid at the top the amount ofliquid at the interface could be controlled, and, thence, the amount ofreaction with Inconel®. A combination of layout scheme and thermal cyclewas employed to arrive at the best bonding conditions which yielded adefect-free interface.

The atmosphere used for the bonding process is very critical as far asthe properties of the hard-faced material are concerned. The effects ofvarious atmospheres were evaluated prior to the bonding process.Tungsten carbide contains 6.7% carbon. The use of air or hydrogen as theatmosphere would decarburize the tungsten carbide and seriously impairsits properties. The use of nitrogen, on the one hand, is not feasibleeither as it would react with the boron in the braze alloy, and traceamounts of oxygen found in the nitrogen have a tendency to oxidizeInconel®. Hence, the use of an inert atmosphere (argon) or vacuum isrecommended for such a system. All the experiments in this study werecarried out in a vacuum, as high purity inert atmospheres like argon aredifficult and costly to obtain.

Bonding Results

In the first set of experiments, the Ni braze formed the interlayer asshown in the schematic in FIG. 1. Adjustments in the thermal cycle weremade based on the results obtained in the bonding operation. FIG. 13shows the microstructure of a cross-section of the tungstencarbide-nickel braze composite hard-faced onto Inconel®. The sample wasprepared by heating the material at a heating rate of 10° C./min to1100° C. with a hold for 30 minutes. The sample was subsequently cooledto room temperature. Even though the tungsten carbide-nickel brazecomposite coating is fully dense, a crack persists throughout theinterface. The presence of excess nickel braze liquid at the interfacewas also observed.

The excess liquid present at the interface causes the delamination tooccur during the cooling operation. During cooling, the tungstencarbide-nickel braze composite, the excess liquid, and the Inconel®undergo shrinkage. The shrinkage rate is slower for the coating ascompared to the excess limit due to the presence of tungsten carbide.This restricted shrinkage is termed as restrained shrinkage. Thisinfluences the crack to occur at the composite coating-excess nickelbraze liquid interface and leads to delamination. A schematic of thisphenomenon is illustrated in FIG. 14.

In an attempt to reduce the interaction between the nickel braze andInconel® leading to formation of a eutectic melt, the hold time wasreduced. FIG. 15 shows the cross-section of a sample bonded with reducedhold time (i.e., 10 minutes). The amount of excess liquid at theinterface decreases, however, it is not sufficient to preventdelamination. The cooling rate was also decreased to 2° C./min to reducethe thermal stress build-up. However, the crack still persisted as shownin FIG. 16. Based on these observations, the layout scheme was changedin order to control the excess liquid at the interface.

The preferred sample arrangement is depicted in FIG. 2. The basicconcept behind this kind of arrangement is that any excess liquid whichis present subsequent to infiltration is left on top, and can be removedand reused. The nickel braze acts as a liquid reservoir on melting. Theinfiltration of the nickel braze liquid is aided by capillary forces andgravity. Once full infiltration is achieved, the nickel braze liquidreacts with the Inconel® to provide interfacial bonding. An additionaladvantage of this kind of arrangement is that, once full infiltration isachieved and the reaction at the interface is initiated, the liquiddevelops a back pressure which prevents further infiltration of theliquid. This in effect controls the amount of excess liquid at theinterface, thus minimizing chances of interfacial cracking.

FIG. 17 demonstrates a sample which was obtained by heating the samplewith the above layout. The setup was heated at 10° C./min to 1100° C. invacuum for 15 minutes. The cooling rate employed was 5° C./min to 700°C. with a hold time of 1 hour. This was essential to provide a stressrelieving treatment prior to cooling to room temperature. This wasfollowed by furnace cooling to room temperature. A fully densifiedcomposite coating bonded onto Inconel® with a defect-free interface isobserved in FIG. 17. The microstructure of the tungsten carbide-nickelbraze composite coating is shown in FIG. 18. The final tungsten carbideloading was evaluated using an image analyzer, and is approximately 75%by volume (85% by weight). The maximum thickness of the compositecoating after bonding is about 2.286 mm [0.09 inch]. Subsequent to thebonding process, the sample was machined using a diamond grinding wheel.The use of any other wheel is not recommended as it may lead to tungstencarbide pull-outs.

The present project aimed at the production of a tungsten carbide-nickelbraze composite coating onto Inconel® 600. An entirely new concept ofsimultaneous composite development and bonding to Inconel® usingpressureless sintering in one single operation was successfullyachieved. This process employs the nickel braze liquid as the bondingagent between the Inconel® and the coating as well as a cementing agentfor densification of tungsten carbide. The process is commerciallyviable, and is inexpensive as compared to other hard-facing methods likechemical vapor deposition, laser cladding, high pressure diffusionbonding, etc. The process makes use of infiltration of a tungstencarbide tape-cast sheet which has controlled porosity with nickel brazeliquid, and subsequently reaction of the nickel braze with Inconel® toprovide interfacial bonding. The bonding process employed a temperature1100° C. (i.e., above the liquids temperature of nickel braze) with ahold time of 15 minutes in a vacuum. A hold at 700° C. for an hour wasprovided during the cooling cycle for relieving thermal stresses. Thisprocess yielded a fully-dense coating bonded onto Inconel® with adefect-free interface.

References

The following references were referred to in the brackets of theforegoing sections:

Ref. No. Title  1. Aronsson and H. Pastor, “Cemented Carbide Powders andProcess- ing”, Powder Metallurgy.- An Overview [at pp. 312-330].  2. B.Aronsson, “Influence of Processing on Properties of Cemented Carbide”,Powder Metallurgy, Vol. 30, No. 3 (1987) [at pp. 175-181  3. Prakash, H.Holleck, F. Thummier, and G. E. Spriggs, “Tungsten Carbide CementedCarbides with Improved Binder Alloys”, Towards Improved Performance ofTool Materials, edited by R. S. Irani, E. A. Almond, and F. A. Kirk, TheMetals Society, London (1982) [at pp. 118-121].  4. D. Moskowitz, M. J.Ford and M. Humenik, JR., “High Strength Tungsten Carbides”,International Journal of Powder Metallurgy, Vol. 6, No. 4 (1970).  5. D.Moskowitz, “Abrasion Resistant Iron-Nickel Tungsten Carbide”, ModernDevelopment in Powder Metallurgy, Vol. 10 (1977), pp. 543-551.  6. R. J.Nelson and D. R. Milner, “Densification Processes in the TungstenCarbide Cobalt System”, Powder Metallurgy, Vol. 15, (1972) [at pp.347-′)63].  7. R. F. Snowball and D. R. Milner. “Densification Processesin the Tungsten Carbide-Cobalt System”, Powder Metallurgy, Vol. 11, No.21 (1968) [at pp. 2340].  8. R. Warren and M. B. Waldron,“Microstructural Development During the Liquid Phase Sinterin ofCemented Carbides 1. Wet- tability and Grain Contact”, PowderMetallurgy, Vol. 15, No. 30 (1972) [at pp. 166-180].  9. R. M. German,Liquid Phase Sintering, Plenum Press, New York (1985). 10. T. Farooq andT. J. Davies, “Tungsten Carbide Hard Metal Cemented with Ferroalloys”,The International Journal of Powder Metallurgy, Vol. 27, No. 4 (1991)[at pp. 347-355]. 11. T. Farooq and T. J. Davies, “A Study ofAlternative Matrices for tungsten carbide Hardmetals.” PM Into the'90s-International Conference on Powder Metallurgy, Vol. 2, Institute ofMetals, London (1990) [at pp. 388-394]. 12. T. Farooq and T. J. Davies,“Preparation of Some New Tungsten Carbide Hardmetals.” Powder MetallurgyInternational, Vol. 22, No. 4 (1990) [at pp. 1216]. 13. R. K.Viswanadham, P. G. Lindquist, and J. A. Peck, “Preparation andProperties of tungsten carbide-(Ni, Al) Cemented Carbides”, Science ofHard Materials, edited by R. K. Viswanadham. D. J. Rowcliffe, and J.Gurland, Plenum Press, New York. 14. B. Roebuck et al., “Partitioning ofMolybdenum Between Carbide and Binder Phase in tungsten carbide/Nicemented carbides infil- trated with Ni-Cr-Mo Alloys”, Journal ofMaterial Science Letters, Vol. 5 (1986) [at pp. 473-474]. 15. B. Roebucket al., “Infiltration as a Method for Producing tungsten carbideHardmetals with Co and Ni Alloy Binder-Phases”, Interna- tional Journalof Refractor-j, and Hard Metals, Vol. 3, No. I (1984) [at pp. 35-41].16. O. Knotek et al., “Carbide-Matrix Reactions in Wear ResistantAlloys”, Science of Hard Materials, R. K. Vishvanadham et al (eds.),Plenum Press, NY. 17. P. M. Roberts, “Brazing Cemented Carbide”, MetalConstruction (Jan. 1987) [at pp. 12-18]. 18. J. H. Chen et al., “TheMetallurgical Behavior During Brazing of Ni-Base Alloy Inconel 600 toSi3N4 with Ag71Cu27Ti2 Filler Metal”, Journal of Material Science, Vol.28 (1993) [at pp. 2933-2942]. 19. S. Kang et al., “Issues inCeramic-to-Metal Joining: An Investiga- tion of Brazing a SiliconNitride-Based Ceramic to a Low- Expansion Superalloy”, Ceramic Bulletin,Vol. 68. No 9 (1989) [at pp. 1608-1617]. 20. J. R. MC'Dermid et al.,“The Interaction-of Reaction-Bonded Silicon Carbide and Inconel 600 witha Nickel-Based Brazing Alloy”, Metallurgical Transactions, Vol. 20A(Sept. 1989) [at pp. 1803-1810].

Modifications

The present invention contemplates that many changes and modificationsmay be made. For example, the particular substrate material is notlimited to Inconel® or 15-5 PH stainless steel. Indeed, the substratematerial can be readily changed. Similarly, the coating composition cancontain a number of components, such as hard-facing particulatematerial, a binder, a plasticizer and a solvent. These variousingredients are typically mixed under a vacuum, cast onto a hydrophilicpolyethylene sheet, dried for thirty hours in air, and then debinded anddensified.

The manner by which the coating composition is applied to the substrateis readily variable, and is not limited to the specific techniquesdiscussed herein.

Therefore, while the preferred steps of practicing the improved methodhave been shown and described, and several modifications thereofdiscussed, persons skilled in this art will readily appreciate thatvarious additional changes and modifications may be made withoutdeparting from the spirit of the invention, as defined anddifferentiated by the following claims.

What is claimed is:
 1. The method of applying a particular material to ametallic substrate, comprising the steps of: removing impurities from asurface of said substrate; forming a coating composition having a brazematerial and at least one particulate material, said braze materialbeing a nickel braze alloy having a melting temperature of about930-970° C. and being less than the melting temperature of saidparticulate material; applying said coating composition to saidsubstrate surface; and creating a diffusion bond between said substrateand coating composition by sequentially heating said coating compositionand substrate to a first temperature above the melting temperature ofsaid braze material but less than the melting temperature of saidparticulate material, holding said coating composition and substrate atsaid first temperature for a first period of time, cooling said coatingcomposition and substrate from said first temperature to a secondtemperature of about 700 degrees C., which is below the meltingtemperature of said braze material, holding said coating composition andsubstrate at said second temperature for a second period of time, andthen cooling said coating composition and substrate to room temperature,such that substantially no defects are created between said coatingcomposition and substrate when said heated composition and substrate arecooled due to the presence of excess braze liquid degenerated duringsuch heating; whereby said particulate material and said braze materialcomprise a layer on said substrate surface.
 2. The method as set forthin claim 1 wherein impurities are removed from said substrate surface bythermal decomposition.
 3. The method as set forth in claim 1 whereinimpurities are chemically removed from said substrate surface.
 4. Themethod as set forth in claim 1 wherein impurities are electrolyticallyremoved from said substrate surface.
 5. The method as set forth in claim1 wherein impurities are removed from said substrate surface by oblationby ions.
 6. The method as set forth in claim 1 wherein impurities areremoved from said substrate surface by oblation by particles.
 7. Themethod as set forth in claim 1 wherein impurities are removed from saidsubstrate surface by oblation.
 8. The method as set forth in claim 1wherein impurities are ultrasonically removed from said substrate. 9.The method as set forth in claim 1 wherein impurities are removed fromsaid substrate surface by fluxing.
 10. The method as set forth in claim1 wherein said coating composition is applied to said substrate surfaceby spraying.
 11. The method as set forth in claim 1 wherein said coatingcomposition is applied to said substrate surface by dipping saidsubstrate surface into said coating composition.
 12. The method as setforth in claim 1 wherein said coating composition is painted onto saidsubstrate surface.
 13. The method as set forth in claim 1 wherein saidcoating composition is applied to said substrate surface by tape-casing.14. The method as set forth in claim 1 wherein said coating compositionincludes an organic binder.
 15. The method as set forth in claim 1wherein at least one particulate material is selected from the groupconsisting of tungsten carbide, titanium carbide, vanadium carbide,titanium diboride, zirconium oxide, hafnium carbide, molybdenum carbide,chrome boride, silicon carbide, diamond, hafnium diboride, zirconiumcarbide and tantalum carbide.
 16. The method as set forth in claim 1wherein the composition of said coating composition and substratechanges in a continuous manner in a direction normal to said substratesurface in the vicinity of said substrate surface.
 17. The method as setforth in claim 1 wherein the composition of said coating composition andsubstrate changes in a non-abrupt manner in a direction normal to saidsubstrate surface in the vicinity of said substrate surface.
 18. Themethod as set forth in claim 15 wherein said at least one particulatematerial is tungsten carbide, and wherein the amount of said tungstencarbide in said coating composition after diffusion bonding is about 85%by weight.
 19. The method as set forth in claim 1 wherein the thicknessof said coating composition after diffusion bond is created is greaterthan 2.0 millimeters.
 20. The method as set forth in claim 1 whereinsaid first temperature is about 1100° C.
 21. The method as set forth inclaim 20 wherein said first time is about 15 minutes.
 22. The method asset forth in claim 20 wherein said second time is about 1 hour.
 23. Themethod as set forth in claim 1 wherein said substrate and coatingcomposition are heated in a vacuum.
 24. The method as set forth in claim1 wherein said substrate and coating composition are heated in an inertatmosphere.